1. Field of the Invention
The present invention relates to rack shafts having a rack tooth row and steering systems including the same.
2. Description of the Related Art
For example, as described in Japanese Patent Application Publication No. 2013-6517 (JP 2013-6517 A), a rack shaft having a rack tooth row is applied to a steering system that steers wheels of a vehicle. For example, as described in Japanese Patent Application Publication No. 2014-79769 (JP 2014-79769 A), a rack shaft has a rack tooth formation portion, a non-rack tooth portion, and a connection portion. The rack tooth formation portion refers to a portion having a rack tooth row formed therein, and the non-rack tooth portion refers to a portion having no rack tooth row formed therein. The connection portion refers to a portion that connects the rack tooth formation portion and the non-rack tooth portion.
In the rack tooth formation portion of the rack shaft, an excess material of a shaft member escapes in the axial direction during plastic working such as forging. This tends to result in underfill in the rack tooth located at an end in the axial direction of the rack tooth row. Such a rack tooth is therefore less likely to have desired accuracy.
Such a rack tooth that does not have desired accuracy cannot be used to mesh with a pinion. Accordingly, in the connection portion of the rack shaft, what is called a sacrificed tooth, namely an ineffective rack tooth, is formed next to the effective rack tooth at the end in the axial direction of the rack tooth row. Moreover, a padding portion that connects the peripheral surface of the non-rack tooth portion and the bottom of the ineffective rack tooth is formed next to the ineffective rack tooth. As used herein, the “ineffective rack tooth” refers to the rack tooth that is not at all contacted by pinion teeth of the pinion that meshes with the rack tooth row, and the “effective rack tooth” refers to the rack tooth that is at least partially contacted by the pinion teeth. The padding portion also improves release properties of a punch from a die in a mold for use in forging.
In the conventional rack shafts, the rack tooth formation portion and the non-rack tooth portion are connected by the connection portion having the ineffective rack tooth and the padding portion formed therein. Accordingly, there is a limit in reducing the axial length of the rack shaft.